Abscisic acid is one of plant hormones such as auxin, gibberellin, cytokinin, ethylene etc. Since abscisic acid was found in 1963, its physiological actions including abscission layer formation, dormancy induction, germination suppression, flowering suppression, bolting (flower stalk development) suppression, transpiration suppression, aging promotion, and stress resistance (e.g. cold resistance enforcement) came to be known. Although it is assumed that abscisic acid generally exhibits a growth suppressing action as described above, it was recently found that similar to other plant hormones, abscisic acid exhibits both promoting and suppressing effects depending on its concentration, and for example it promotes plant growth to raise the yield at low concentration (Nakabori et al., Bulletin of the Aomori Agricultural Experiment Station in 1991 (1992)). Further application to the promotion of thickening and maturing fruits (Japanese Patent LOP Publication Nos. 264,005/1992, 264,006/1992 and 264,007/1992), prevention of flowers or unmatured fruits from falling (Japanese Patent LOP Publication No. 139,911/1993), growth promotion for agricultural products (Japanese Patent LOP Publication No. 178,705/1993) or flowering promotion (Japanese Patent LOP Publication No. 186,303/1993) is known.
However, abscisic acid is expensive and among optical isomers of abscisic acid, natural type one demonstrates higher effects, and thus abscisic acid is not practically used. Recently, a method of producing natural type abscisic acid by culturing a microorganism of the genus Botrytis was developed, but it is hard to say that this method is satisfactory (Japanese Patent LOP Publication Nos. 296,696/1988, 296,697/1988 and 60,590/1990). Some reports have been made of its organic synthesis, but there remain problems with a large number of steps, costs, stereoselectivity (Helv. Chim. Acta, 71, 931 (1988); J. Org. Chem., 54, 681 (1989); and Japanese Patent LOP Publication No. 184,966/1991).
Out of those compounds which relate to the plant growth regulator of the present invention, a free carboxylic acid and methyl ester derivative are described in the above literatures as intermediates for chemically synthesizing abscisic acid, but it is not disclosed that such intermediates exhibit abscisic acid-like physiological actions.
On one hand, brassinosteroids are a group of ubiquitous compounds present in plants, and exhibit specific physiological growth actions, such as growth promotion, fertilization and fructification promotion, cold resistance enforcement, promotion for thickening fruits etc., and promotion for germinating or rooting of seeds or cuttings.
However, it was not known that an intimate mixture of abscisic acid or an abscisic acid-like substance and a brassinosteroid exhibits a synergistic effect on plant growth regulation.